This invention relates generally to reversible vapor compression refrigeration (heat pump) systems. More particularly, the invention relates to a method and apparatus for warming the outside heat exchanger of a heat pump system periodically when the system is operating in the heating mode in order to remove frost and ice accumulations. The typical heat pump system for heating and cooling an enclosed space comprises a compressor, an outside heat exchanger, an inside heat exchanger, expansion devices and a flow reversing valve. In the cooling mode, the inside heat exchanger functions as the evaporator in an otherwise standard vapor compression refrigeration cycle. In the heating mode, refrigerant flow through the two heat exchangers is reversed and the outside heat exchanger functions as the evaporator A fan circulates air from the space to be
During operation in the heating mode, frost and ice can form on the external surfaces of the outside heat exchanger of a heat pump system, degrading system performance. To eliminate the buildup, the heat pump is periodically placed in a defrost mode of operation in order to melt any frost and ice from the heat exchanger.
One widely used method of heat pump defrosting is to operate the system for a short time with the refrigeration flow aligned as in the air conditioning mode but with the outside fan shut off. The inside fan continues to operate with heat being removed from the space to be heated and used to defrost the outside heat exchanger.
Small window mounted or room air conditioning heat pump systems commonly employ a single motor to operate both the inside and outside fans. The two fans are both mounted on the rotor shaft of the motor and cannot operate independently. Therefore, the defrosting method described above cannot be used with heat pump systems having a single fan motor. In such systems, defrosting of the outside heat exchanger may be accomplished in a number of ways including:
Passive defrost - The refrigeration compressor does not operate while the outside heat exchanger temperature us allowed to rise to the outside temperature, which must Passive Defrost - The heat pump refrigeration cycle does not operate while the outside heat exchanger lengthy process and result in high operating costs due to the use of electric heat. The requirement for supplemental electric heaters also raises the unit initial cost.
Fan off defrost - The compressor operates but the fans do not. Thus the heat for defrosting comes only from that added to the system by the compressor. This is also an inefficient and lengthy process during which the system cannot provide any space heating capability. The method may not be effective at low outside temperatures and may also require a supplemental orifice and associated solenoid valve, thus increasing the cost of the unit. Further, liquid refrigerant floodback, with resultant possibility of compressor damage, is likely when using this method.
Because both of the above defrosting methods may be ineffective at low outside air temperatures, their use is rendered unsuitable except for heat pump systems designed for mild climates.
What is needed therefore is an effective and economical means for defrosting, under a wide range of climatic conditions, the outside heat exchanger of a heat pump system having heat exchanger fans that cannot be operated independently.